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Abstract

Ipsimanual and contramanual simple reaction time (RT) elicited by unilateral visual stimuli are often conceptualized in terms of a fastest route between visual system activation and motor output. Indices of brain function based on this scheme, such as interhemispheric transmission time (IHTT), fail to consider the anatomical complexity of the pathways involved and the multiple activation routes possible. These issues were explored by examining the spatiotemporal. dynamics of visuomotor activation and relations between physiology and behavior using high-density event-related potentials (ERPs). Data analyses involved scalp current density (SCD) mapping (including animations) and exploratory spatiotemporal dipole analysis to estimate the intracranial sources of activation foci. The data suggest that bilateral posterior and central activations occur when the same hemisphere receives direct visual input and generates motor output violating the intrahemispheric processing assumption underlying RT estimation of IHTT. The distribution of stimulus-related activity over frontal cortex was also investigated. Right frontocentral activation was independent of input visual field or motor response requirement. An additional study demonstrated that the distribution of simple RT reflects changes in the magnitude and timing of motor cortex activation. SCD mapping showed a sequence of activations that were presumably located within prefrontal cortex, premotor cortex, supplementary motor area (SMA) and primary motor cortex (M1) of the hemisphere contralateral to the responding hand. SMA and M1 contralateral to the responding hand showed activation changes as a function of RT. Laplacian waveforms, calculated from the centers of presumed SMA and M1 activations and were lag cross-correlated with the forearm EMG to estimate the corticomuscular delay and determine the primary location of motor output activations. Two main findings were obtained: (1) given similar timing of activation onset, greater activation magnitude was associated with faster RTs; and (2) the slowest RTs were associated with delayed onset of motor cortex activation. These results show that behavioral observations such as RT cannot unambiguously be used to index the timing of information processing stages, since a &sim;100 ms range of RTs may be associated with differences in the magnitude, but not the onset timing, of activation of movement-related cortical regions.*.;*Dissertation includes CD-ROM which will only will work on Windows-based PCs.